The invention relates to a method for determining the image clarity of a surface of a sheet, strip or similar, comprising reflection of an image on the surface, detection of a virtual image of the image projected onto the surface and the processing of the detected virtual image of the image projected onto the surface into a signal that is presented as a measure for the image clarity of the surface.
At the same time the invention relates to an apparatus for carrying out such a method.
In the refrigerator, washing machine and automotive industries it is desirable to know the degree of image clarity of a sheet given that the sheet used for the external parts of refrigerators and washing machines, or of cars must be particularly flat and smooth because of the optical quality after painting. Indeed it has been found that there is a quality aspect of the sheet used to be found in this image clarity of the surface of the steel sheet used. In this connection image clarity is the term known in the above-mentioned industries for describing the degree of distortion of a virtual image of an image projected onto the surface. In the case of a surface with a relatively poor image clarity, the outward appearance of the steel sheet used looks somewhat irregular after painting. This phenomenon is known as orange peel. For reasons of cost it is desirable at the earliest possible processing stage and preferably before painting to select the steel sheet for suitability for any exterior parts of white goods or cars which are visible. Consequently it is desirable to have a related measuring method which is reliable and reproducible and which excludes any subjective elements.
Use is known, for example of the reflectometer by ATI Systems Inc., model 1792, whereby a controlled light beam is projected onto the surface being examined and the light reflected is received by a receiver unit and converted by a processor unit into a signal that is a measure for the brilliance of the surface.
It has been found that this known method and apparatus is not very discerning and has little value in predicting the orange peel aspect in a sheet to be used for exterior parts. In addition, the value of the information obtained with the known method and apparatus depends greatly on an operator's interpretation of the data obtained.
The object of the invention is to create a reliable method for determining the image clarity of a surface which method has a good prediction value and does not depend on subjective aspects. To this end the method in accordance with the invention is characterised in that the image reflected onto the surface is composed of a number of distinctive light spots and in that the number of distinctive light spots is determined from the virtual image detected, which number determines, at least in part, the magnitude of the signal that is produced as a measure for the image clarity of the surface.